A continuously varying
trait is a heritable characteristic that exhibits a range of phenotypic
variation within a population, as opposed to discrete traits that are either
present or absent. Examples of continuously varying traits include height,
weight, skin color, and eye color. These traits are influenced by multiple
genes and environmental factors, and their expression can be affected by
interactions between genes and the environment. The study of continuously
varying traits is important in fields such as genetics, evolutionary biology,
and ecology, as it provides insights into the mechanisms underlying the
variation and adaptation of organisms to their environments.
The term ‘variation’ describes
the characteristics shown by organisms belonging to the same natural population
or species. A study of the phenotypic differences in any large population shows
that two forms of variation occur, discontinuous and continuous.
Discontinuous Variation
Variation in this case
produces individuals showing clear cut differences with no intermediates
between them, such as ABO blood groups in humans, pea seeds round or wrinkle,
wing length in Drosophila and sex in animals and plants. Characteristics
showing discontinuous variation are usually controlled by one or two major
genes which may have two or more allelic forms and their phenotypic expression
is relatively unaffected by environmental conditions. Since the phenotype
variation is restricted to certain clear cut characteristics, this form of
variation is also known as qualitative inheritance.
Continuous Variation
Many characteristics in
a population show a complete gradation from one extreme to the other without
any break. For example traits like height, weight, intelligence and skin color
in humans and grain color. Continuous variation is also known as quantitative
inheritance and its genetic basis are the polygons or polygenic inheritance.
Polygenic Inheritance
Polygenic inheritance
(Gk. Polys, many, and L. genets, producing) occurs when one trait is governed
by several genes occupying different loci on the same homologous pair of
chromosomes or on different homologous pairs of chromosomes. Each gene has a
contributing and non-contributing allele. The contributing allele is
represented by a capital letter and the noncontributing allele is represented
by a small letter. Each contributing allele has a quantitative effect on the
phenotype and therefore the allelic effects are additive.
For example, H. Nilsson
Ehle studied the inheritance of seed color in wheat. Genes at three different
loci determine seed color. After he crossed plants that produced white and dark
red seeds, the F1 plants were allowed to self-pollinate. Each of the F2 produced
seeds having any one of the following seven colors.
The alleles responsible
for the color of the seeds are represented by dots in the diamond shapes. The
number of capitals is represented by coloring in the dots. Any capital has the
same effect on the color of the seed. Notice that each capital (contributing
allele) has a small but equal quantitative effect and that this accounts for
the various degrees of color and the observed range in F2 phenotypes.
In polygenic
inheritance, each contributing allele adds to the phenotype. A bell-shaped
curve of phenotypes is also due to environmental effects.
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| Polygenic Inheritance |
Human
skin color: It is a quantitative trait which is
controlled by three to six gene pairs. The greater the number of pigment
specifying genes, the darker the skin. Exposure to sun can affect the skin
color and increase the number of phenotypic variations.
Human
Height: This trait is controlled by many pairs of genes at
different loci. There is a wide range in variation of human height. Tallness is
recessive to shortness. If there is more dominant alleles in the genotype,
shorten will be the individual and if there is more recessive allele in the
genotype taller will be the person. Environment and nutrition can effect height
resulting in many more phenotypes.
Histogram
The bar chart in the
figure plots the proportions of the phenotypes for wheat seed color, human skin
color. The vertical bars that are the shortest represent categories with least
number of dominant ge.ne. The bar that is the tallest represents the category
with greatest number of dominant genes. The reverse is in the case of human
height as tallness is recessive to shortness. If a graph line is drawn all
around the bar a bell shaped curve is obtained. Such curves are typical of
populations that show continuous variation in a trait.
Frequency histograms
illustrate variations. A frequency histogram is a simple gram. The horizontal
or X axis indicates the range of different phenotypes of a trait within a
population. The vertical or Y axis indicates the number of individuals or their
percentage in the population as seen in the figure.
The tongue rolling
ability is due to a single dominant gene. It is a discontinuous variation
inherited in simple Mendelian fashion. Its frequency diagram forms asymmetric
distribution curve, with much greater frequency of phenotype at one end than
the other.
Human height is a
continuously varying trait. Its frequency histogram forms a smooth bell shaped
normal distribution curve.
